1. Field of the Invention
This invention relates to logic circuits having non-volatile memory. More specifically, it relates to reset signals sent to memory controllers for NAND-type flash memory and power supplies for NAND-type flash memory.
2. Description of the Related Art
Flash is one type of non-volatile memory. EPROM (Erasable Programmable Read Only Memory) can be programmed and erased enabling them to be re-used. Erasure is accomplished using an UV (Ultra Violet) light source that shines through a quartz erasing window in the EPROM package.
There also are OTP (One Time Programmable) EPROMs, sometimes called OTPROMs (One Time Programmable Read Only Memory), that are identical to an erasable EPROM but lack an erasing window to reduce costs. To reduce their cost these EPROMs come in a windowless plastic carrier, which is cheaper than the costly ceramic package required for the erasing window. They can be programmed one time only.
An EEPROM (Electrically Erasable Programmable Read Only Memory) is similar to an EPROM but the erasure is accomplished using an electric field instead of an UV light source. This eliminates the need of a window. Usually, EEPROM refers to a device that requires a programmer or special voltage to program it.
A flash EPROM is similar to an EEPROM except that large blocks of flash EPROMs are erased all at once while a regular EEPROM can erase one byte at a time. However, with flash EPROMs, in-circuit writing and erasing is possible because no special voltages are required. To accomplish in-circuit operation, one can write special application software routines. Flash EPROMs are also called nonvolatile memory.
Flash memory is a type of EEPROM chip. It has a grid of columns and rows with a cell that has two transistors at each intersection. The two transistors are separated from each other by a thin oxide layer. One of the transistors is known as a floating gate, and the other one is the control gate. The floating gate's only link to the row, or wordline, is through the control gate. As long as this link is in place, the cell has a value of 1. To change the value to a 0 involves a process called Fowler-Nordheim tunneling.
Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge, usually 10 to 13 volts, is applied to the floating gate. The charge comes from the column, or bitline, enters the floating gate, and drains to a ground.
This charge causes the floating-gate transistor to act like an electron gun. The excited electrons are pushed through and trapped on other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. A device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than 50 percent of the charge, it has a value of 1. When the charge passing through drops below the 50-percent threshold, the value changes to 0. A blank EEPROM has all of the gates fully open, giving each cell a value of 1.
A CompactFlash® card is a small, removable mass storage device. First introduced in 1994, CompactFlash (CF) cards weigh about a half an ounce and are the size of a matchbook. They provide complete PCMCIA-ATA functionality and compatibility. CF data storage cards are also available using magnetic disk technology (Microdrive). With the built-in controller, a wide variety of low cost flash technologies can be used. The built-in controller lowers costs further by reducing costs in the host device and allowing defective flash chip cells to be mapped out, thus increasing flash chip yields. Microdrives provide the lowest cost data storage solution for capacities of 340 MB and above.
The BIOS (Basic Input/Output System) chip of a personal computer is perhaps the most common form of flash memory. However, removable solid-state storage devices are becoming increasingly popular. SmartMedia and CompactFlash cards are both well-known, especially as “electronic film” for digital cameras. Other removable flash memory products include Sony's Memory Stick, PCMCIA memory cards, and memory cards for video game systems.
Two main technologies dominate the non-volatile flash memory market today: NOR and NAND. The NAND-based solution is ideal for high capacity data storage, while NOR is best used for code storage and execution, usually in small capacities. There are many differences between NAND and NOR flash. One of them is the interface. NOR flash is basically a random access memory device. It has enough address pins to map its entire media, allowing for easy access to each and every one of its bytes. NAND devices are interfaced serially via a rather complicated I/O interface, which may vary from one device to another or from vendor to vendor. The same eight pins convey control, address and data information. NAND is typically accessed in bursts of 512 bytes; i.e., 512 bytes can be read and written at a time (similar to hard drives). This makes NOR ideal for running code, while NAND is best used as a data storage device (hard drive/block device replacement).
A typical storage-type CompactFlash contains many NAND flash memory units and an interface controller. Because of the nature of the serial interface, the controller handles the data transfer via a serial interface so that it is transparent to the high level interface. (i.e. CompactFlash card interface). Data corruption or loss can occur inside the CompactFlash memory if the power to the memory card is disrupted or a reset signal is received during a write cycle because the built-in controller may not have completed the write cycle and acknowledged it as a valid data. The corruption may not be recovered correctly and cause the system to hang if the error recovery/correction (ECC) circuit inside the built-in controller mis-detects this situation. One possible way of preventing such data corruption is to employ a hard disk drive as an alternative to flash non-volatile memory. However, flash memory has a number of advantages over hard disk drives: flash memory is noiseless; it allows faster access; it is smaller in physical size; it is lighter; and, it has no moving parts. Accordingly, a solution which solves this problem while permitting flash memory to be used for non-volatile data storage in situations where power loss during a write cycle may be encountered is critically needed.